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Looks like Clickjacking is the web-security buzzword of the week (month?), since Robert "RSnake" Hansen and Jeremiah Grossman decided to cancel their OWASP talk, drawing an aura of mystery around the whole issue and its magnitudo.

Nevertheless some info and speculations have been percolating, and even if the precise details of the attacks proposed by those two researchers are still embargoed, especially because of the serious and not necessarily obvious implications worrying Adobe, a certain awareness about the general technique and the possible countermeasures does circulate now. In Jeremiah's and RSnake's words:

Think of any button on any Web site, internal or external, that you can get to appear between the browser walls, wire transfers on banks, Digg buttons, CPC advertising banners, Netflix queue, etc. The list is virtually endless and these are relatively harmless examples. Next, consider that an attack can invisibly hover these buttons below the users’ mouse, so that when they click on something they visually see, they actually are clicking on something the attacker wants them to. [...]
Say you have a home wireless router that you had authenticated prior to going to a [malicious] web site. [The web site] could place a tag under your mouse that frames in a single button an order to the router to, for example, delete all firewall rules.

In other words, the attack is thrown by a malicious web page embedding objects, possibly from a different site, such as framed documents or plugin content (Flash, Silverlight, Java...) which may lead to unwanted results if clicked by the current user (e.g. a "Delete all messages" button in your webmail or an advertisement banner in a click fraud scheme). Using DHTML, and especially CSS, the attacker can disguise or hide the click target in several ways which go completely undetected by the user, who's easily tricked into clicking it in a more or less blind way.

JavaScript increases the effectiveness of these attacks hugely, because it can make our invisible target constantly follow the mouse pointer, intercepting user's first click with no failure. We can however imagine a few less effective but still feasible scriptless scenarios, e.g. covering the whole window with hidden duplicates of the target or overlaying an attractive element of the page, likely to be clicked (e.g. a game or a porn image link), with a transparent target instance.
Nevertheless, as RSnake puts it,

[...] the best defense against clickjacking attacks is to use Firefox with the NoScript add-on installed. Users running that combination will be safe, said Hansen, against “a very good chunk of the issues, 99.99 percent at this point.”

That's true because attacking from an untrusted page not allowed to run JavaScript is highly impractical, but also because NoScript by default prevents Java, Silverlight and especially Flash content, which seem so far the most dangerous clickjacking targets, from being embedded on non-whitelisted pages.

But what about that damned 0.01%? That's given by framed documents, most notably IFRAMEs. For a live and benign example of what you can do with IFRAME-based clickjacking, look at NoScript's "install now!" widget, which gets dynamically overlayed by the addons.mozilla.org install page: they're positioned so that if you click on the orange button you automatically install from AMO, skipping the security notification bar you would get on any other site. This "clickjacking" of mine has been there for a long time (since AMO V3, IIRC), and it heavily relies on JavaScript.

But even if an IFRAME-based attack was carefully crafted to work without JavaScript, NoScript would still provide effective protection, scoring a perfect 100% by RSnake's standards. You just need to enable the Plugins|Forbid <IFRAME> option, and cross-site IFRAMEs will be blocked by default on untrusted pages: they will need a confirmation to be activated, therefore "blind clicks" become impossible. Zone 365 and Hardware Forums created a short video tutorial about this setting. If you want to be protected even against unlikely attacks thrown from a trusted site included in your whitelist, check Plugins|Apply these restriction to trusted sites as well: embedded objects (plugin content and frames) get blocked on every site, but you can enable any of them on the fly by clicking on its placeholder.

A final recommendation is reading this Michal Zalewski's contribution, which covers the IFRAME case only but is very generous with mitigation proposals, both for web developers and browser vendors: by the way, his browser fix proposal #4 is almost identical to current NoScript's Forbid <IFRAME> option, and simpler variants of proposal #3 are being explored as default features in NoScript development builds since version 1.8.1.7.